Controlled fabrication of Carbon Nanotubes : material of the future

September 11, 2018 John S. Bulmer (University of Cambridge)
Carbon Nanotubes 3D model

As a scientist at the University of Cambridge, I’m closely involved in a fascinating project on Carbon Nanotubes. In cooperation with Bronkhorst, we are working on a reactor to control the fabrication of this exceptionally strong and conductive material. Let me explain more about this subject and why I consider Carbon Nanotubes to be a material of the future.

History and future of Carbon Nanotubes (CNT)

In the beginning, carbon came in three molecular forms:

  • diamond
  • graphite
  • amorphous carbon
Suddenly, in the mid-1980’s, a new molecular form of carbon surfaced in research and ignited the multidisciplinary field of nanotechnology. This all carbon molecule, Buckminsterfullerene, is a nanometre-sized cage of carbon atoms with a molecular structure that resembles a football.

3D model of Buckminsterfullerene
3D model of Buckminsterfullerene

A few years later, another molecular carbon cousin came to light: carbon nanotubes (CNT). Similar to Buckminsterfullerene, the football structure is vastly elongated into a nanometre-wide tube with length millions of times greater than its diameter. Captivating scientific attention; CNT’s strong carbon bonds with its ordered molecular structure make it the strongest material ever made. Electrons glide down CNT’s effortlessly, as stable one-dimensional conductors, which makes CNT’s electrical conductivity four times greater than copper and with a maximum current carrying capacity 1,000 times greater than copper.

By the early 2000s, researchers created processes to fabricate textiles composed of CNT’s with densely packed and aligned microstructure. Initially, the bulk properties of CNT textiles lagged well behind the exciting properties of their individual molecules. After steady incremental improvement, the state-of-the-art CNT fibre is as strong as conventional carbon fibre and about four times more conductive. With continued development we expect CNT fibres that are substantially stronger than conventional carbon fibre with an electrical and thermal conductivity greater than traditional metals like Copper and Aluminium.

Application of Carbon Nano Tube fibres is in strain-resistant textiles (protective clothing, bullet-proof vests), composites, construction compounds (ceramics, lighter car bodies) and cables because of their strength. Using carbon nanotubes could have enormous impact on day-to-day life, similar to the way plastics changed the world in the mid-20th century.

Carbon Nanotubes (CNT) at the University of Cambridge

Our laboratory invented a production process that not only creates Carbon Nanotubes in industrially competitive volumes, but does so with unparalleled graphitic perfection into a macroscopic textile with aligned microstructure, all in one production step. This production process is intrinsically simpler than other fibre production processes such as conventional carbon fibre and Kevlar.

The floating catalyst chemical vapour deposition reactor (F-CVD)  that is used for this process just requires a carbon source (toluene), a catalyst source (ferrocene) and a Sulphur based promotor (thiophene), which are mixed together and fed into a 1300°C tube reactor by a carrier gas (hydrogen). A floating CNT cloud is formed. Mechanically extracting the CNT cloud out of the tube reactor condenses the cloud into a bulk fibre with aligned microstructure. This is called “CNT spinning”. Specially protected personnel, also known as “the spinner”, mechanically extracts the CNT cloud into a fibre.

Consistent reactor control however, is challenging. The CNT material properties vary substantially between runs and the relationship between controlled and uncontrolled reactor input parameters are not fully understood yet.

Control of the Carbon Nanotubes Reactor

Our program seeks to implement a robust feedback loop to control the reactor’s CNT material properties. Every reactor input variables and output variables, which are specifically selected CNT material properties, are automatically measured and recorded into a database; from the outside weather, to the operating personnel, to the age of the tube, to the precursor concentrations, gas flows, etc.. The database is continually data mined for correlations, parameter interaction, and multidimensional linear regression models that statistically predict reactor behaviour using the data exploratory software JMP™.

Figure 1. Statistical model for the material’s G:D ratio
Figure 1. Statistical model for the material’s G:D ratio

For example, figure 1 shows a statistical model for the material’s G:D ratio, this is the ratio between graphite (G) and graphitic defects (D) from Raman spectroscopy, which indicates the degree of graphitic perfection. The model is a function of various reactor input parameters that were found the most statistically significant to the G:D ratio. On the horizontal axis in the plot below, there are the predicted G:D values of the model and, on the vertical, the actual measured vales. In a perfect model with perfect control, we would expect a straight 45 degree line. Clearly, the data points are widely spread along the red line, which indicates a low level of reactor control.

The setup here involved simply mixing the precursors together (toluene, ferrocene, and thiophene) and injecting the solution into a hydrogen carrier gas via a simple gear pump. It became evident a more sophisticated system was required for greater reactor control.

Bronkhorst solution for control of the Carbon Nanotubes Reactor

Figure 2 shows our improved system. Separate liquid precursors are now independently controlled with Bronkhorst Coriolis instruments (mini CORI-FLOW series). The Coriolis mass flow meters give precise mass flow rates without the need of recalibration between different precursors, which greatly facilitates trying out different CNT recipes. Bronkhorst is the only one who succeeded in applying the well-known high-precision Coriolis principle to an extremely small scale by applying MEMS technology.

The flow rates are in the range up to 200 g/h for toluene and even below 100 mg/h for thiophene. Hydrogen carrier gases are controlled by robust, plug-and-play Bronkhorst mass flow controllers. Finally, the precisely metered precursors are vaporized and combined with the controlled hydrogen carrier gases with vaporizer technology.

Figure 2. Carbon Nanotubes Reactor Scheme
Figure 2. Carbon Nanotubes Reactor Scheme
Figure 3. Chemical vapour deposition reactor is much more effective
Figure 3. Chemical vapour deposition reactor is much more effective

With this new and more sophisticated instrumentation, statistical modelling of the floating catalyst chemical vapour deposition reactor is much more effective. Here, the actual versus predicted values for the graphitic perfection are much more agreeable, as is shown in figure 3. This model has substantially less noise, which means the reactor’s response is predictable and repeatable. So far, with this controllable and well modelled reactor system, we have more than doubled typical CNT production rates and tripled the degree of graphitic crystallinity.

Stay tuned! With Bronkhorst and other important commercial, academic, and government partners we hope to surpass conventional carbon fibre soon!

Bronkhorst information

If you are active in reactor technology, do not hesitate to contact us for solutions for your processes. Please contact us for more information.

  • Read more about MEMS technology that is used for the research in Carbon Nanotubes in the previous blog of Wouter Sparreboom.

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